Explanation: she lied to him, she erased the person sitting next to her but didn't erase his hand

This seems to describe a situation where someone tried to manipulate a photo to hide or lie about the presence of another person. She edited the image to remove the person sitting next to her, likely to cover up their presence, but forgot to erase the person's hand, leaving evidence of the manipulation.

In other words, her attempt at deception was flawed because the hand in the photo gives away the fact that someone was originally there.